Sending failed:java.net.NoRouteToHostException:operation timed out:connect why is it so and how

i am succeeded in developing the project of the mail client..butwhat the thing is that i amreceiving an error..as when i enter the email addresses and all the stuff and when i send themsg..it gives the msg to meSending mailand then after sometimeSending failed:java.net.NoRouteToHostException:operation timed out:connectwhy is it so and how can it be solved?